The nociceptive events that may occur locally on the ocular surface are quite unique to the eye and, in the majority of cases, are best described as ocular surface irritation and discomfort. Symptoms unique to the eye include foreign body sensation, photosensitivity, photophobia and an actual sensation of dryness of the eye. Foreign body sensation is essentially identical to a “something in the eye” feeling, which may occur in varying degrees from discomforting but tolerable to intolerable in extreme cases. These foreign body sensations and other mild nociceptive events also promote a more rapid rate of blinking (nictation) and possibly tearing. This may be accompanied by a mild stinging or burning sensation. Photophobia and photosensitivity are unique to the eye and this hypersensitivity to light results in squinting and eye closure to relieve this unpleasant sensation.
Treatment of ocular pain and discomfort induced by dry-eye disease is an unmet medical need. For relief of post-surgical pain, the current standards of care are: (1) topical steroids, but long term use is associated with severe side effects; (2) NSAIDs, these have some effects only on surgical pain and are useful as pretreatments but are much less effective in reversing ongoing conditions. This is because NSAIDs only block the new synthesis of all prostanoids, leaving pre-existing ocular prostanoids in ongoing medical conditions to still interact with their receptors to cause pain and irritation. On the other hand, IP antagonist could treat ocular pain by blocking IP receptor, bypassing the need to attenuate prostanoid synthesis.
Artificial ocular surface wetting agents and lubricants provide some relief for ocular discomfort. The afflicted person may eventually present at the physician's office for treatment. There is no proven therapeutic intervention available to attenuate the nociceptive stimulation and neurotransmission at the ocular surface and resultant discomfort sensations.
The therapeutic modality that provided relief from ocular discomfort described herein is a prostacyclin (IP) receptor antagonist. In a model where ocular surface discomfort produced by mild corneal abrasion was reflected as an increased nictation rate, post-treatment with a prostanoid IP receptor antagonist essentially produced significant relief from ocular surface discomfort and reduced the nictation rate. It is also considered that a prostanoid IP receptor antagonist may be combined with ocular surface lubricants to provide further relief from ocular surface discomfort.
Nociceptors are situated on nerve endings and are the first element in communicating diverse chemical, thermal, and mechanical stimuli to the CNS (spinal cord and brain). Nociceptors roles differ greatly, with functions ranging from sensing blood pressure and CO2 changes to mediating pain. The dull aching or sharp pain associated with injury, inflammation, infection and cancer is divided into two types. These are somatic pain arising from internal tissues and visceral pain arising from internal organs. In internal organs and tissues, the neuronal elements involved in initiating pain responses are polymodal nociceptors, which respond to local neurotransmitters. The initial neurotransmission is then relayed by nerves for central processing and translation into a perceived sensation. Peripheral nociception is yet a further category. Entirely different and unique sensations occur in peripheral nociception, which is confined to those surfaces exposed to the environment; ocular surface and skin and orifices (nasal and anal). These unique sensations include itch and perception of cold and hot.
In the domain of peripheral nociception, there is further divergence and differentiation. The nociceptive stimulation-response repertoire in the cornea possesses unique attributes. The neuronal responsive elements, the pharmacology of corneal nociception, and the quality and gradation of the perceived sensations following corneal insult or injurious stimuli, are unique. Nociceptive events associated with the cornea/ocular surface are those that elicit a sensation that may be discomforting, irritating, but painful only in the more extreme cases. Such responses essentially follow a graduation from discomforting/irritating and tolerable to painful and intolerable. Stimulation of corneal nociceptors (mechanoreceptors, chemoreceptors, thermoreceptors) elicit sensations that would not occur in other tissues. Provided below is an abbreviated compendium of the unique spectrum of events associated with corneal nociception.                1. Unique sensations experienced as a result of corneal nociception: photosensitivity, photophobia, dryness, foreign body sensation and tearing.        2. Corneal sensory nerve endings subserving unique sensations in the cornea:                    (a) Chemoreceptors; corneal receptors that mediate discomfort and irritation caused by environmental pollutants (Belmonte et al., 1999; Lang et al., 2008; Callejo et al., 2015);            (b) Mechanoreceptors; corneal receptors that mediate a pricking sensation (Belmonte et al., 2015);            (c) Thermoreceptors (cold): corneal receptors that mediate feeling of dryness (Parra et al., 2014), cooling, irritation (Belmonte et al., 1999); and,            (d) Polymodal nociceptors; Mediate sensory responses from all irritant/noxious stimuli and mediate hyperalgesia and pain (burning, stinging) (Belmonte et al., 2015).                        3. Receptor pharmacology of corneal surface nociception: Piezo 2 receptors mediate responses to mechanical forces; TRPV1/TPRPA1 receptors mediates responses to heat and chemical stimuli; TRPM8 receptors mediate the response to cold; ASIC receptors mediate the response to acids (Callejo et al., 2015; Belmonte et al., 2015). Non-steroidal anti-inflammatory agents are exert corneal analgesia by inhibiting nociceptors (Chen et al., 1997; Acosta et al., 2007).        
Corneal nociception is unique on all levels. For this reason, in a litany of somatic and visceral pain conditions cited in various patents, corneal nociception is not mentioned and peripheral nociception is virtually ignored (U.S. Pat. No. 6,184,242; U.S. Pat. No. 6,596,876; U.S. Pat. No. 6,693,200; and U.S. Pat. No. 7,141,584, which are hereby incorporated by reference). This is likely because of the potential involvement of mechano- and chemo-nociceptors, in addition to polymodal nociceptors, in peripheral nociception. Nasal and anal nociception are ignored and cutaneous nociception is only included as psoriasis, where psoriatic pain is manifest with rheumatoid arthritis (Chang et al., 2011). The lists do not include ocular pain, discomfort, or other nociceptive conditions originating from the cornea/ocular surface. These are omitted despite the fact that conjunctivitis, a well-known and common condition of the conjunctival tissue, is cited in said patent citations. Conjunctivitis is included in a paragraph dedicated to uses of prostanoid IP antagonists for treating inflammatory pain (U.S. Pat. No. 7,141,584). Thus, it has thereby been defined as somatic pain.
The conjunctiva is one of the anatomically closest tissues to the cornea. Whereas the cornea is clear and with no blood supply, the conjunctiva is a vascularized tissue that can become inflamed and swollen in response to allergic and other inflammatory stimuli. Although the origin of allergic and infectious conjunctivitis is conjunctival tissue, in conjunctivitis there can be sensory symptomology all over the ocular anterior segment, similar to referred pain that may occur with visceral pain. Thus, conjunctivitis could also be arguably considered as akin to not only somatic pain but also visceral pain, according to this criterion.
The symptomatic manifestations of ocular surface and ocular anterior segment diseases are complex and unique to the eye in many respects. This is consistent with sensory receptive elements adapting to and differing according to tissue function. Allergic and infectious diseases usually originate from the conjunctiva. Itch and soreness are prominent symptoms of allergic and infectious conjunctivitis and these neurosensory phenomena likely arise from the conjunctiva. However, studies on conjunctival neurotransmission are very few in number. The technical difficulties of defining sensory receptive fields and isolating nerve fibers emanating from the conjunctiva cannot be surmounted. In short, humans can describe their perceived sensations but nerve conduction cannot be measured and the converse is true in laboratory animals. From these limited studies, it appears that nociceptor populations in the cornea and conjunctiva may mediate quite different sensory modalities. In a study on cold perception, the conjunctiva perceived only a cooling sensation whereas the corresponding corneal sensation included irritation (Acosta et al., 2001). This clearly delineates nociceptor functions in the cornea and conjunctiva.
Certain sensations are, however, common to diseases of the cornea and conjunctiva. These are foreign body sensation and photosensitivity/photophobia. Both are symptoms totally unique to the eye. Foreign body sensation is ubiquitous and may be described as grittiness, sandiness, or a “something in the eye” feeling. Foreign body sensation is discomforting and elicits a desire to rub, wipe, close and/or irrigate the eye. An involuntary response is to increase the blinking (nictation) rate. If severe, foreign body sensation can be intolerably uncomfortable.
The ocular surface neurosensory manifestations may be stratified as follows.                1. Ocular surface discomfort/irritation. This is very common and affects a very significant proportion of the population. Ocular surface discomfort appears to predominantly arise from the cornea. The corneal epithelial layer is densely innervated and is readily activated by physical and chemical insults to the corneal surface and the overlying tear film. Such corneal irritation may arise from drying of the tear film or inadequate tear secretion, immunologically based dry eye disease, and environmental pollutants and contaminants. These insults to the corneal epithelial surface stimulate the nociceptor populations resulting in feelings of dryness, foreign body sensation, irritation and a resultant general discomfort. This degree of nociception may be described as tolerable. Continued dryness or environmental insult may worsen the condition by damaging the corneal epithelium. Persons experiencing ocular surface discomfort typically find some relief by using artificial lubricants. An analgesic drug that would ameliorate the neurosensory activation and associated discomfort would be desirable to achieve more sustained therapy.        2 Ocular pain. This would be best regarded as analogous to the pain associated with inflammatory diseases such as rheumatoid arthritis, degenerative disorders such as osteoarthritis and cancer. A similar or greater level of ocular pain would be caused by alkali or acid burn, penetrating or gross corneal injury, severe microbial infection, or diseases of the ocular anterior segment such as uveitis or severe ocular hypertension. The afflicted individual will typically present in the physician's office or emergency room within 24 hours. A description of intolerable would be applicable to ocular pain.        3 Itch. This is confined to the conjunctiva and is a separate sensation. In all but transient episodes the individual seeks medical treatment.With the exception of treating post-surgical inflammation and pain, the entire spectrum of corneal neurosensory disorders represents an un-met medical need for which there are no commercially available drugs.        
A number of neurotransmitters have been proposed as corneal nociceptive mediators. TRPV1 receptors have been widely considered as transducing corneal nociceptive events. Several lines of evidence would support this. TRPV1 receptors are abundant in the cornea (Murata et al., 2006) although not all are associated with polymodal nociceptors (Chen et al., 1997). The TRPV1 receptor stimulant capsaicin activates corneal neurosensory units (Chen et al., 1997) and produces a behavioral response (eye wiping) in laboratory rodents indicative of a nociceptive sensation (Gonzalez et al.; Bates et al., 2010). In humans, capsaicin induces a sharp sensation of pain (Dupuy et al., 1988). Calcium antagonists have also been reported to reduce capsaicin induced ocular pain (Gonzalez et al., 1993). Regardless of these evidences, TRPV1 antagonists such as capsazepine and the widely available Ca2+ channel blockers such as diltiazem are not used clinically to treat ocular surface pain and discomfort. TRPV1 inactivation by resininferatoxin (Bates et al., 2010) has not received clinical acceptance for treating corneal neurosensory conditions. Other putative nociceptor mediators such as biogenic amines and neuropeptides have been implicated in corneal pain but this has not translated into clinical utility so far.
Local anesthetics are used to alleviate corneal pain but are used sparingly for rapid but temporary relief. The reason being that local anesthetics prevent any sensation of actual damage or foreign bodies in the exterior of the eye and depress corneal reflexes and blinking. Undetected solid matter on the corneal surface would cause or exacerbate corneal abrasion and heighten corneal nociceptive activity. This would be potentially catastrophic.
The only other therapeutic modality that has found limited utility in treating ocular surface nociception are the Cyclo-oxygenase inhibitors (COXIBs). Cyclo-oxygenase inhibitors (COXIBs) are aspirin-like drugs and are widely as used for treating inflammation, pain, and hyperpyrexia. Their therapeutic actions are due to inhibition of prostanoid biosynthesis. COXIBs are widely used for analgesia and hyperpyrexia and are almost invariably given by mouth. The therapeutic of COXIBs in the eye is much more limited and the route of administration is not oral.
In the eye, COXIBs are used as post-surgical anti-inflammatory agents with analgesic properties and as adjuncts to steroid therapy in treating uveitis. The ophthalmologic practice of COXIB use differs somewhat from that followed in general therapeutics. This is in spite of the fact that the utility of orally administered COXIBs for treating pain is incontrovertible. In ophthalmology, COXIBs are almost invariably applied topically to the ocular surface and are not given by mouth for treating post-surgical inflammation and pain. This is despite the fact that COXIBs are available “over-the-counter” and are eminently affordable. The mechanistic basis of COXIB induced ocular analgesia appears to diverge markedly from that associated with typical systemic therapy. This provides a ready explanation for the topical administration preference in ophthalmology.
The clinical effectiveness of COXIBs used for ophthalmological purposes has been questioned on the basis of their limited efficacy and slow onset (Coppens et al., 2002). From the cylco-oxygenase inhibition perspective, prostaglandins (PGs) are endogenous constituents of the cornea (Urquhart et al., 2015). Moreover the cornea has no capacity to metabolically inactivate PGs (Cheng-Bennett et al., 1990) and, since the cornea is an avascular tissue, very little capacity to remove PGs from corneal tissue. This would result in very long PG residence times in the cornea. Since the effects of COXIBs depend on prevention PG biosynthesis, the efficacy of COXIBs should not necessarily be apparent until endogenous PGs are replaced by newly synthesized PGs. The effects of COXIBs in the eye are, therefore, more apparent in therapeutic pretreatment and subchronic intervention over days for surgical trauma. Mechanistic studies on COXIB effects on corneal nociception would support the intuitive contention that an alternative analgesic mechanism must exist. In the cornea, COXIBs have been reported to exhibit an alternative analgesic mechanisms of action that is independent of cyclo-oxygenase inhibition and PG biosynthesis. The COXIBs ketorolac, diclofenac, flubiprofen, and nepafenac (Chen et al., 1997; Acosta et al., 2007) all directly attenuated the responsiveness of corneal polymodal nociceptor fibers. Further, diclofenac and flurbiprofen directly inhibit ASIC induced neurosensory transmission (Voilley et al., 2001). This is of significance since ASIC nociceptors are involved in pain (Wemmie et al., 2013) and ocular surface discomfort (Callejo et al., 2015). Inhibition of ocular surface nociception may represent effects that are made manifest after the tissue concentrations sufficient to inhibit cyclo-oxygenase enzymes have been far exceeded. Nevertheless, the doses used in ophthalmologic nociception studies reflect those used clinically and, therefore, likely represent the in-life clinical situation. Direct inhibition of TRP and ASIC nociceptors by COXIBs seems a more plausible mechanistic explanation than inhibition of prostanoid biosynthesis.
Considering pre-formed and resident PGs as an impediment to successful treatment of corneal nociception by inhibition of cyclo-oxygenase inhibition using COXIB drugs, this therapeutic mechanism of action merits serious consideration. The identity of PG receptors possibly involved in corneal nociception has not been systematically elucidated. COXIB s have an advantage in that they suppress PG biosynthesis and thereby produce global inhibition of PG effects in the cornea. However, since PGs exist preformed in the cornea and COXIB effects depend on blocking de novo PG biosynthesis, prevention of PG receptor stimulation may represent a better therapeutic strategy. Restated in a different way, a prostanoid receptor antagonist may block the activity of preformed pro staglandins in the cornea by directly blocking their interaction with their target receptors. Prostanoid receptor pharmacology is discussed below.
The prostanoids are oxygenated fatty acids with potent and diverse biological activities. They are biosynthesized from arachidonic acid by cyclo-oxygenase enzymes, the intermediate endoperoxides then being converted to a range of different prostanoids by specific prostaglandin synthase enzymes. The major biologically active prostanoids are prostaglandins D2, E2, and F2α, prostacyclin (PGI2), and thromboxane A2. These prostanoids exert their biological effects by interacting with a series of receptor proteins, which preferentially interact with one of the major prostanoids, as follows. Thus, prostaglandin D2 preferentially interacts with DP1 and DP2 receptors, prostaglandin E2 with EP1-4 receptors, prostaglandin F2α with FP receptors, prostacyclin with IP receptors and thromboxane A2 with TP receptors (Woodward et al., 2011).
All of the major prostanoids have been implicated in pain and inflammation (Kawakarni et al., 2001; Ueno et al., 2001; Kunori et al., 2009; Woodward et al., 2011; Gatta et al., 2012; Kanda et al., 2013). This may explain why inhibitors of the cyclo-oxygenase enzymes 1 and 2 are extensively used as analgesics and receptor selective antagonists are not. Although potent and selective antagonists for each of the individual prostanoid receptors have been developed over many years, none have found favor as clinically useful analgesics. Indeed, the prostanoid EP1 receptor was initially viewed as the most likely nociceptor involved in pain (Woodward et al., 2011) and selective EP1 antagonist drugs were developed and studied clinically. These have not found clinical utility as analgesics.
Consideration of prostacyclin and its target receptor (IP) receptor as a mediator in pain could be viewed as counter-intuitive because of its exceptionally short biological half-life (Cho and Allen, 1978). IP antagonists have been studied in three of the four categories of “pain”, namely somatic, visceral, peripheral and neuropathic. Experimental evidence exists for a role in somatic inflammatory pain, visceral pain, neuropathy induced nociception, and hyperalgesia. No evident inclusion of peripheral nociception is apparent. In models of inflammatory pain, gene deletion and IP receptor antagonists have been reported as effective analgesics (Bley et al., 2006; Woodward et al., 2011). In IP receptor knock-out mice, the pain associated with the writhing response to acetic acid was dramatically reduced (Murata et al., 1997). This signifies a role in visceral pain (Ohishi et al., 1999; Huang et al., 2010). In addition to analgesic activity in acute models, IP receptor antagonism also reduces the pain and inflammation in models of hyperalgesia and chronic arthritis (Pulichino et al., 2006). Prostacyclin has also been implicated in the central processing of neurotransmission (Doi et al., 2002; Nakae et al., 2005; Schuh et al., 2014). In one study, however, although the IP receptor agonist iloprost indicated a functional role for prostacyclin in exciting dorsal root ganglia, virtually all prostanoid receptors sensitized rat dorsal root ganglia (Nakae et al., 2005). Nevertheless, such nociception produced by IP agonists in rodent cellular and living animal studies would be complimentary to the IP antagonist studies. A detailed anatomical analysis of prostacyclin in pain transmission is difficult because all of the reports to date do not distinguish between central or local transmission of nociceptive responses. All of these studies were performed in mice or rats, which several authors acknowledged in the titles of their articles. In a clinical study in human rheumatoid arthritis subjects, the IP receptor agonist iloprost was found to improve markers of inflammation and reduce joint pain swelling, tenderness and pain (Gao et al., 2002). This constitutes an absolute contradiction of results obtained in rat and mouse studies and renders rodent models of pain unreliable. The limited predictive value of mice and rats compared to primates is a documented concern with respect to clinical translation (Vierboom et al., 2008). For the reasons described above and the unique nociceptive qualities of the ocular surface described below, non-human primates were chosen as the species of choice for the purpose of translational reliability. Moreover, a known clinical condition was replicated in the monkeys, namely mild corneal abrasion.
The proposed involvement of prostanoid IP receptors in mediating and processing inflammatory pain responses has been its origins in rodent models of inflammation. These models result in inflammatory pain that exhibits certain characteristics akin to those that are frequently encountered in rheumatoid arthritis and other inflammatory diseases. For example, pressure on the affected area or limb exacerbates and/or creates pain. Applying pressure by standing on the affected load-bearing limb(s) is painful and devices have been invented to automatically monitor pressure applied by small animal footpads to a touch sensitive plate; less pressure being applied by the swollen and/or painful limb. Movement of affected limbs exacerbates or creates local pain, this can often be severe to the point where fear of movement becomes a very distressing factor in persons with inflammatory and neuropathic pain.
Nociceptive events on the ocular surface produce an entirely different repertoire of behaviors and sensations to those encountered in inflammatory pain from, for example, rheumatoid conditions. Squeezing and/or pressing the eye actually relieves ocular pain. An increased nictation rate also results. Rubbing the eyes to remove the perceived foreign body also occurs, especially in animals where the foreign body sensation cannot be construed as a “phantom” syndrome. The cornea is an avascular tissue, therefore tissue swelling and leukocyte infiltration do not contribute to nociceptive responses as would be the case in solid vascularized tissues and joints.
Prostaglandins participate in the inflammation associated with cataract surgery and pain associated with corneal refractive surgery. Cyclo-oxygenase inhibitors (COXIBs) such as ketorolac are used as surgical adjuncts and for photoradialkeratotomy (PRK), whereas selective prostanoid receptor antagonists have not been employed for these therapeutic indications. Cyclo-oxygenase inhibitors (COXIBs) have not found extensive use for ocular surface disorders, beyond being used as post-surgical adjuncts. One factor that may influence the use of COXIBs in ocular surface disorders is the long duration of prostanoid residence in the cornea; the cornea has little capacity to enzymatically deactivate prostaglandins (Cheng-Bennett et al., 1990) and the cornea is avascular. Therefore, once biosynthesized in the cornea, prostaglandins have a very long residence time. It follows that the utility of cyclo-oxygenase inhibitors may be compromised in post-treatment dosing regimens, where prostaglandins are already resident at elevated levels in the cornea: these drugs can prevent prostanoid biosynthesis but cannot affect prostanoids already formed and present. In contrast, a receptor antagonist can directly compete with the prostaglandins already resident in the cornea for their receptors and thereby reverse their effects. This is an important therapeutic consideration. The patient usually presents at the physician's office or emergency room with a pre-existing condition that is affecting ocular wellbeing. It is, therefore, of considerable therapeutic significance that a prostanoid IP receptor antagonist can actually reverse the nociception in a primate model of ongoing discomfort/irritation.